linux-kernel - Re: [PATCH] RTC: Avoid races between RTC alarm wakeup and suspend.

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Message-Id: <201207302301.49972.rjw@sisk.pl>
Date:	Mon, 30 Jul 2012 23:01:49 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	NeilBrown <neilb@...e.de>
Cc:	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RTC: Avoid races between RTC alarm wakeup and suspend.

On Monday, July 30, 2012, NeilBrown wrote:
> 
> If an RTC alarm fires just as suspend is happening, it is possible for
> suspend to complete and the alarm to be missed.
> 
> To avoid the race, we must register the event with the PM core.
> 
> As the event is made visible to userspace through a thread which is
> only scheduled by the interrupt, we need a pm_stay_awake/pm_relax
> pair preventing suspend from the interrupt until the thread completes
> its work.
> 
> Signed-off-by: NeilBrown <neilb@...e.de>
> 
> --
> This makes the pm_wakeup_event() call in cmos_interrupt unnecessary as it
> provides suspend protection for all RTCs that use rtc_update_irq.

Care to remove the call in cmos_interrupt(), then?

> I think the pm_stay_awake//pm_relax is needed - just pm_wakup_event() is 
> theoretically not sufficient.
> 
> This is because there is no guarantee (that I know of) that the workqueue
> thread will actually get scheduled before 'suspend' takes over.

I think you are right.

Thanks,
Rafael


> diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
> index eb415bd..9592b93 100644
> --- a/drivers/rtc/interface.c
> +++ b/drivers/rtc/interface.c
> @@ -582,6 +582,7 @@ enum hrtimer_restart rtc_pie_update_irq(struct hrtimer *timer)
>  void rtc_update_irq(struct rtc_device *rtc,
>  		unsigned long num, unsigned long events)
>  {
> +	pm_stay_awake(rtc->dev.parent);
>  	schedule_work(&rtc->irqwork);
>  }
>  EXPORT_SYMBOL_GPL(rtc_update_irq);
> @@ -844,6 +845,7 @@ void rtc_timer_do_work(struct work_struct *work)
>  
>  	mutex_lock(&rtc->ops_lock);
>  again:
> +	pm_relax(rtc->dev.parent);
>  	__rtc_read_time(rtc, &tm);
>  	now = rtc_tm_to_ktime(tm);
>  	while ((next = timerqueue_getnext(&rtc->timerqueue))) {
> 

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